(1) Field of the Invention
This invention relates, in the case where a synthetic resin container whose wall has more than four layers is subjected to blow or orientation blow molding, to an injection molding method for a multi-layer bottomed parison subjected to injection molding in advance.
(2) Prior Art
In synthetic resin containers having a thin-wall thickness formed by blow or orientation blow molding, the construction of the wall portion thereof is more than two layers to compensate for defects of synthetic resins from which the container is formed, for example, such as a heat resistance, gas barrier property and the like.
For forming a multi-layer container, it is generally preferable to form a three-layer construction which interposes between resins, of which a container is formed, a resin as an intermediate layer. This three-layer container, that is, a container comprising an inner layer and an outer layer which both layers being formed of the same resin, and an intermediate layer formed of the other resin, can be molded from a three-layer bottomed parison which was subjected to injection molding by use of a nozzle of double construction.
However, even though the bottomed parison is in a molten state when being subjected to injection molding, the two different resins used therefor do not adhere to each other and are easily separated by an external pressure.
Therefore, in bottles filled with carbonated drinks or the like, the carbonated gases which permeate into the inner layer are impaired from permeation into the outer layer by the presence of an intermediate layer having gas barrier properties interposed between the inner and outer layers, and the carbonated gases are often stagnate between the inner and intermediate layers generating bubbles.
As a means for preventing the phenomenon as just described, an attempt has been made to form adhesive layers on both sides of the intermediate layer to make sections thereof five layers.
Molding of a five-layer bottomed parison by injection molding is disclosed in Japanese Patent Application Disclosure No. 56-501082. According to the technique disclosed in the aforesaid patent application, a three-layer molding is basically provided which comprises a means for independently starting a first polymer flow serving as an inner surface layer of a parison, a second polymer flow serving as an outer surface layer and a third polymer flow serving as a center layer. In the case of a five-layer molding, a quintuple nozzle is used in which a fourth polymer flow if introduced between the third and first polymer flows and a fifth polymer flow is introduced between the third and second polymer flows.
In the above-described conventional technique, the inner surface layer and outer surface layer of a parison are formed from two different polymer flows, the central first flow and outer second flow, and between these two polymer flows, the third, fourth and fifth polymer flows which constitute the other layers introduced. Thus, a nozzle used therefor should be a multiple nozzle provided with polymer flows equal in number to that of layers of a bottomed parison as required.
Unlike an extrusion molding nozzle in which each flowpassage is open to outside, an injection molding nozzle has a construction wherein each flowpassage is merged at one nozzle hole irrespective of the number of flowpassages. Therefore, there is a limit in the number of flowpassages, and it has been extremely difficult to securely mold a five-layer bottomed parison from a single nozzle by providing, within the nozzle, flowpassages in the number corresponding to that of layers of bottomed parisons. Moreover, even if such a multiple nozzle should be obtained, it would require a specific molding technique to injection-mold a bottomed parison without losing layers even after hollow container having a five-layers in section, each layer having an even wall thickness, has been formed by blow or orientation blow molding.